my $ary_ref = $sth->fetchall_arrayref();
if ($DBI::errstr) {
  print "Error detected: $DBI::errstr\n";
  return;
}
else {
  if (ref($ary_ref) eq 'ARRAY') {
    # go on to process the array reference.
  }
  else {
    # do something with Data::Dumper
  }
}